As spectral bandwidth becomes an ever more valuable commodity for radio communication systems, techniques are needed to effectively use the available bandwidth. MIMO wireless technology is one such technique. MIMO uses multiple transmitters and multiple receivers to increase data throughput, e.g., to increase data transfer bandwidth, and/or to improve the signal-to-noise (SNR) ratio, e.g., to reduce the signal fading. With respect to increased data transfer bandwidth, the MIMO increases the capacity of a data channel by increasing the number of receive (RX) and transmit (TX) antennas to take advantage of signal multipathing. For example, multiple pairs of transmit/receive antennas can be deployed in the communication system to transmit multiple spatial streams (e.g., wireless signals or symbols). These data streams can be transmitted simultaneously and on a single frequency by using proper multiplexing (MUX) at the TX side of the communication system. The combination of multiple, independent data paths increases the throughput of a MIMO system. At the RX side of the MIMO system, the transferred data are recombined into the original data stream using suitable demultiplexing (deMUX). Typically, an increase of the number of RX and TX antenna pairs linearly increases the throughput of the MIMO system.
Furthermore, the MIMO systems can improve SNR ratio of the wireless data. For example, the same wireless signals can take multiple paths from the TX to the RX as the wireless signals reflect from objects that are in the path of, or adjacent to, the TX and RX. In absence of MIMO, these multiple paths would generally increase signal interference at the RX. However, with MIMO systems these additional paths can be utilized to improve the SNR of the wireless signals. For example, in a relatively simple implementation, a MIMO system may identify and select the strongest signal among the multiple available signals, while discarding other signals. In more advanced implementations, a MIMO system may accept multiple wireless signals as representing different versions of the same wireless signal having slight delays because of the reflection of the signal or because of differing distances traveled by the various paths from the TX antenna to the RX antenna. Generally, multiple versions of the same signal undergo statistically independent fading because the signals travel along different physical paths. When these different versions of the signal are summed up (or otherwise combined using proper adjustments for differing time delays or phase changes) into a resulting signal, the SNR improves due to statistical independence of the signal fading.
However, for practical MIMO systems, it is difficult to properly characterize different data channels (e.g., to calibrate gains and delays from one antenna to another). Furthermore, data channels may behave differently based on the direction of data transfer (e.g., whether the antenna sends the signal or receives it, or whether the wireless signal is transferred from antenna 1 to antenna 2 or from antenna 2 to antenna 1). Accordingly, a need remains for improved characterization of data channels in MIMO systems.